(1) Field of the Invention
This invention relates to steam turbines, and more particularly to a method of and a system for controlling thermal and centrifugal stresses produced in the rotor of a steam turbine during operation of the steam turbine.
In recent years, the need to meet efficiently the ever-increasing demand for electric power has markedly increased the capacity of each steam turbine plant. This has made it necessary to operate a power plant in a manner to cope successfully with variations in the demand for electric power, particularly to deal with the difference in the demand for electric power between daytime and nighttime. Thus, a power plant hitherto operated at base load or with a constant turbine power is nowadays required to frequently effect startup, shutdown and load changes. Operation transients such as startup, shutdown and load changes produce thermal stress of high magnitude in the steam turbine rotor. Thus, in controlling stresses produced in the steam turbine rotor, it is important to control thermal stress produced by the aforesaid operation transients. It is also necessary to take into consideration centrifugal stress which would be produced in the turbine rotor during its rotation. The problem of stress control may be considered to be the problem of preventing excess life consumption of the turbine rotor by controlling operation of the steam turbine or the problem of controlling the life of the turbine rotor. With regard to the analysis of thermal and centrifugal stresses produced in a turbine rotor and the problem of controlling turbine rotor life, a prior paper is cited which is entitled `The Operation of Large steam Turbines to Limit Cyclic Thermal Cracking` by D. T. Timo and G. W. Sarney (ASME Paper No. 67-WA/PWR-4, published in 1967).
(2) Description of the Prior Art
In one method known in the art for controlling stress produced in a steam turbine rotor, the stress produced at the surface of the rotor and the stress produced at the rotor bore are controlled. According to this method, life consumption of the turbine rotor is controlled by taking into consideration low-cycle fatigue caused by thermal stress of the base portions of a high-pressure initial stage disk and a reheating initial stage disk of the steam turbine rotor in respect of the surface of the rotor, and by taking into consideration thermal stress and creep life in respect of the rotor bore. A problem raised with regard to this method has been that brittle fracture of the turbine rotor is not taken into consideration when stress control is effected. Generally, high-pressure and medium-pressure sections of a turbine rotor are formed of material of high high-temperature brittle fracture toughness because they are usually exposed to heat of high temperature. As a result, these sections have low low-temperature brittle fracture toughness. These sections of the turbine rotor are exposed to steam of relatively low temperature when the number of revolutions increases at startup. Thus, in controlling stress produced in the turbine rotor, the low low-temperature brittle fracture toughness of the material of these sections should be taken into consideration. Also, stress control should be effected by taking into consideration high-temperature fracture toughness of the material of a low-pressure section of the rotor during operation of the steam turbine at load, because such material is low in high-temperature brittle fracture toughness.